1. Field of the Invention
The invention relates to a method of processing a signal representing a sequence of two-dimensional images conveyed in sub-sampled form by way of a transmission channel or record carrier and subjected, prior to said conveyance, to different sampling structures on a segmented basis depending on the degree of movement and/or the character of spatial information, said method including the step of receiving the sub-sampled signal from said transmission channel or record carrier sequentially on a field by field basis. The invention also relates to receiving apparatus for use with the above method.
2. Description of Related Art
Such a method of processing and apparatus are disclosed in the papers "Adaptive spatial sub-sampling for HD-MAC" by O.J. Morris and T.I.P. Trew and "Motion compensated interpolation applied to HD-MAC pictures encoding and decoding" by F. Fonsalas, M. Haghiri and P. Hayet which were presented at the 2nd International Workshop on Signal Processing of HDTV held at L'Aquila, Italy between Feb. 29, and Mar. 2, 1988. The method and apparatus described in these papers are for use with a coding system as described in a further paper presented at the same workshop "An HD-MAC coding system" by F.W.P. Vreeswijk, W. Jonker, J.R.G.M. Leenen and J. van der Meer which achieves a bandwidth compression factor of four with little loss in resolution by adapting to the motion in the picture being transmitted. The coding system transmits information taken from one instant in time over four field periods in stationary areas of picture, over two field periods in areas of slow motion, and over one field period where the motion is rapid. These three modes of operation are known as the 80 ms, 40 ms and 20 ms branches for a 50 Hz field frequency, 2:1 interlaced system. An overall compression of 4:1 is required between the 25 MHz luminance bandwidth of the 1250-line high definition camera and the 625-line 6 MHz transmission channel. The system therefore makes a compromise between discarding temporal and spatial resolution as shown in the following example:
______________________________________ System Period Temporal Compression Spatial Compression ______________________________________ 80 msec 2:1 2:1 40 msec 2:1 2:1 20 msec 1:1 4:1 ______________________________________
Thus several different field rates are used for different velocity ranges as follows:
i. In a stationary mode (velocity range: 0-0.5 pixels/40 msec) the field rate is 12.5 Hz and the basic interval is 80 msec. PA1 ii. In a slowly moving mode (velocity range: 0.5-1.5 pixels/40 msec) the field rate is 25 Hz and the basic interval is 40 msec. PA1 iii. In a moving mode (velocity range: above 1.5 pixels/40 msec) the field rate is 50Hz and the basic interval is 20 msec. PA1 i. forming from said received sub-sampled signal a further sub-sampled signal containing the same information but in which the order of the fields is changed compared with that in said received signal, PA1 ii. subjecting said further sub-sampled signal to inter-field and/or intra-field processing, and PA1 iii. forming from the resulting processed signal an output signal in which the order of the fields is restored to the sequence present n said recieved signal.
One possibility is for these temporal branches to use fixed spatial quincunx sub-sampling patterns with filtering optimized for high horizontal and vertical resolution at the expense of diagonal resolution. There will be some spatial structure within pictures for which this is not the most appropriate form of filtering and, therefore, adapting the spatial processing according to the picture content by introducing sub-branches within each branch may give a substantial improvement in quality. Each sub-branch can support different spatial frequencies; the transmitter selects the sub-branch that best represents the frequencies in an area of the picture and signals this to the receiver through the digital assistance (DATV) channel. For this reason the paper by Morris et al proposes that each branch can comprise a number of sub-branches having different sub-sampling patterns. The coding systems described in the papers of Vreeswijk et al and Morris et al and the processing method and apparatus of the latter paper are also contained in our co-pending European patent application No. 88202912.7. This application and the above mentioned papers are incorporated herein by way of reference.
The paper by Morris et al proposes the use of adaptive spatial interpolation to improve the performance of bandwidth reduced systems at the receiver, while the paper by Fonsalas et al purposes the use of motion compensated spatial interpolation for the same purpose. This allows the 40 ms branch to be used in areas having a velocity range above 1.5 pixels per 40 msec. The latter technique provides a problem when interpolating across the boundaries between areas processed prior to transmission through different branches having, respectivey, a low frame rate and a high frame rate which can result in holes being left in the intermediate fields where information is transmitted in only half the fields as is the case in the 40 ms branch. Some estimate is required for this area in order to `run-in` the filters interpolating pixels in the surrounding non-motion-compensated areas. It is not possible to simply insert the information from the previous field, which is the procedure used in non-motion-compensated multi-branch systems, since dislocations might occur in features crossing the area or block boundary. Indeed, the motion thresholds for each branch in non-motion-compensated systems are set so that this dislocation will be imperceptible. It is not advisable to project the missing block back into the previous frame along the motion vector, inserting the sub-sampled pixels within the image into the vacant area in the current field, since the block image might not have pixels in the correct phase with the expected sub-sampling structure or might even include areas transmitted through other branches. One possible solution is to fully interpolate the previous frame and to project the missing block in the current frame back onto this. The resulting image would then be moved into the position of the missing block and sub-sampled with the correct structure. This does not give perfect results since the motion vectors are selected so as to provide a reasonable interpolation when projected to both the preceding and following frames, and the displaced images averaged. If only one image is used then dislocations are introduced due to errors in the motion vectors, introducing spurious structure along the block boundaries. Such a solution is considered to be unacceptable.